Self-tensioning cable drive

ABSTRACT

Disclosed is a self-tensioning endless rope or cable drive system in which a sectional load engaging dolly is used, the dolly sections being joined by a block and tackle reeving of the endless rope so that as the load is picked up, or engaged, by the dolly, a predetermined portion of the force necessary to move the load is transferred to the lower tension portion of the endless rope to thereby prevent slipping of the cable at the area of application of motive driving power thereto.

Oct. 7, 1975 United States Patent [1 1 Traughber, Jr.

[ SELF-TENSIONING CABLE DRIVE [76] Inventor: Charles W. Traughber, Jr.,Box Primary Examiner Leonard Geri 88C, Candler, NC. 28715 Oct. 29, 1974AppL No.: 518,572

Attorney, Agent, or Firm-Woodard, Weikart, Emhardt & Naughton [22]Filed:

[57] ABSTRACT Disclosed is a self-tensioning endless rope or cable drivesystem in which a sectional load engaging dolly is used, the dollysections being joined by a block and tackle reeving of the endless ropeso that as the load is 505 2 2 m 7HM 4:102 2F. H m 4 4 4 M 6 1 HF m 4 .u.c .e "8 L C 0 mm UIF 1]] 2 00 555 [[i picked up, or engaged, by thedolly, a predetermined References Cited portion of the force necessaryto move the load is UNITED STATES PATENTS transferred to the lowertension portion of the endless 74/2429 X rope to thereby preventslipping of the cable at the 7 242 9 X area of application of motivedriving power thereto.

74/2429 X 10 Claims, 5 Drawlng Figures "Hut "fin e a pu ma mmnm rOheTYCN 7 4 4577 9999 1111 I/// 6892 US. Patent Oct. 7,1975 Sheet 1 of43,910,130

US. Patent 0a. 7,1975 Sheet 2 of4 3,910,130

US. Patent Oct. 7,1975 Sheet 3 of4 3,910,130

U.S. Patent Oct. 7,1975 Sheet4 0f4 3,910,130

gamma? SELF-TENSIONING CABLE DRIVE BACKGROUND rope THE INVENTION Endlesswire ropes drives are utilized in many and various applications, oneexample is its use in the brick making industry where it is used to movewheeled containers or carts of heavy ceramic objects such as bricks fromone oven or area of the plant to another.

The transmission of power by endless rope drives, such as elliptical andU-groove drives, depends upon the frictional resistance developedbetween the cable and the drum surface. A driving force is transmittedfrom the surface of the driving pulley to the cable because of thefrictional resistance between the two surfaces. This frictionalresistance(and, hence, the load moving power available) is, of course, adirect function of the tension in the cable and its length of contactwith the drum. The effective length of contact with the driving drum canbe increased by using a drive drum or pulley means which includes aplurality of driven drums with the cable wrapped, in multiple, aroundthe assembly such multiple drum drives and single drum drives all beinggenerically included in the term drive pulley means as used herein.

The tension in the cable, and hence the normal force exerted by it onthe drive drum, will vary throughout its length of contact with the drumand this force will in crease in magnitude from the smaller, or low,tension side of junction of the cable and drum (T to the large, or hightension side of the belt (T When the cable picks up or engages a load itis thus quite important to maintain the proper tension on the lowtension side of the cable and various weight-type and power cylinderdevices, whose action is independent of the magnitude of the load, areutilized in prior art systems for tensioning the cable or belt, that is,for maintaining the desired tension on the low tension side of the belt.The driving force available is dependent upon maintaining the properratio of the larger tension side of the cable (T,) to the smallertension side (T As a load is picked up by the cable, since T thereuponincreases substantially as a direct function of the magnitude of theload, in order to maintain the proper T to T ratio the tension on thesmaller tension side of the cable (T must be increased.

The system and apparatus of the present accomplishes the propertensioning of the cable by utilizing a sectional load pick-up dolly withthe dolly section carrying the load engaging abutment being secured tothe cable-attached, remaining dolly section by means of a block andtackle type reeving of the cable extending between the dolly sections.When a load is engaged, therefore, a predetermined fraction of the forcenecessary to move the load is transferred to the smaller tension side ofthe cable, the magnitude of the fraction being inversely proportional tothe number of passes of the cable making up the block and tackle typeconnection between the dolly sections. The tensioning force transferredto the lower tension side of the cable is, thus, a direct function ofthe size of the load to be moved. no external source of hydraulic orpneumatic power is required for cable tensioning. At no-load op erationresilient means are provided for insuring a minimum tension on the cableto prevent slippage. The tensile stress on the cable system is thusreduced except when a load is engaged. A take-up winch may be providedon one of the dolly sections to compensate for any permanent stretchingdeformation of the cable. A modified form of the dolly, utilizing three,rather than two dolly sections, permits bi-directional operation of thesystem.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic, perspectiveview of an endless rope drive system incorporating the presentinvention.

FIG. 2 is a top plan view of the two-section dolly embodying the presentinvention.

FIG. 3 is a side sectional view taken generally along the line 3-3 ofFIG. 2.

FIG. 4- is a schematic, top view of the dolly structure shown in FIGS. 2and 3 but illustrating, in detail, the reeving between the dollysections.

FIG. 5 is a schematic top view, similar to FIG. 4, but illustrating amodified form of the dolly which utilizes three, rather than two, dollysections.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring initially to FIG. 1,there is shown a multiple drum drive which includes adjacent drivinghubs or drums l0 and 11, both of these drums being driven at the samespeed by a reversible motor (not shown) and through a gear reductionsystem as is customary. The driving drums are rotated in the samedirection and are provided with multiple, matching grooves whichaccommodate passes of an endless belt 12. With the drums rotating in thedirection indicated in FIG. 1, the belt will have a larger tension sideT and a smaller tension side T The use of the dual driving drumsincreases the total are of contact of the belt with the driving drumsthereby increasing the available power for moving loads picked up by thebelt. As is customary the smaller tension side of the belt moves over anidler sheave 13 and over tail sheaves l4 and 15, these serving toreverse the direction of the belt and direct it toward its largertension junction (T with the drum 10. A load engaging member or dolly iscarried by the rope and indicated generally at 16. The dolly isinterposed within a separation in the rope l2 and is formed by two dollysections 17 and 18. The dolly section 17 is secured to the largertension side of the rope indicated at 12a and, assuming that the drivepulley assembly 10 and 11, is rotating in the direction indicated inFIG. 1, the dolly section 17 will be the leading section. The dollysection 18 is provided with wheels 21, the wheels 19 and 21accommodating the dolly for travel along the path of the belt 12 whichis the path over which a load is to move leftwardly as viewed in FIG. 1from, in general, the location of the tail sheaves l4 and 15 to thepulley assembly 10 and 11.

The lower tension portion of the belt, indicated at 12b, moves unbrokenbeneath or adjacent to the dolly 18. The other portion of the lowertension side of the rope, indicated at 12c extends over the dollysection to the dual sheaves indicated generally at 22. The sheaves 22are freely rotatable independently and are carried by the leading dollysection 17. The rope extends between the sheaves 22 and dual,independent sheaves 23, carried by the dolly section 18, in multiplepasses, as will subsequently be described in detail with reference toFIG. 5. After its final excursion around the lower one of the sheaves 23the rope extends to the leading dolly 17 and is attached to the dollysection 17 by means of a take-up winch 24 carried by the leading dollysection. It will be understood that the take-up winch might be locatedon dolly section 18, rather than leading section 17 however, in thiscase there would be one less pass of the rope from the sheaves 23 to theleading dolly section 17. The trailing dolly section further carries aload engaging abutment 26 which is pivotally supported so that it iscapable of engaging a load in only one direction of movement of thedolly, that is, as the dolly moves from right to left in FIG. 1.

Further details of the dolly construction and its function in providinga tensioning force to the smaller tension side of the rope (T,) willbecome evident upon an examination of FIGS. 2-4 now to be described.

Referring to FIGS. 2 and 4, the dolly 16 includes a trailing section 18(mentioned previously with respect to FIG. 1) which includes a plate 31carrying telescoping members 32 which extend longitudinally toward theadjacent dolly section 17. The wheels 21 are journalled in supportinghousings 21a which may be welded or otherwise suitably fastened to theadjacent plate structure. Depending from the plate 31 are dual, groovedsheaves 35a and 35b which are free to rotate on the stub shaft 33, thesheaves carrying peripheral rope or cable grooves 34 and 36,respectively. These sheaves are the counterparts of the sheaves 23illustrated schematically in FIG. 1.

Extending from plate 31 are spaced members 41 (FIG. 2) which journal ashaft 42 carrying the loadengaging abutment or dog 26 previouslyreferred to with reference to FIG. 1. The abutment element 26 iscontoured at 26a so that it is capable of clockwise rotation about theaxis of shaft 42 when engaging a load (such as a pick-up lug, not shown,on the underside of a load conveyor car) when the dolly is moving fromright to left as viewed in FIG. 3, thereby slipping past the pick-uplug. Engagement of the surface 26b with a pick-up lug, as the dollymoves in the opposite direction, however, because dog 26 remains uprightand cannot rotate, will cause the dolly to pick up the load. A springpin 26c, and an additional spring (not shown), urges the dog into itsupright position.

The extending members 32 telescope within tubes 43 rigidly supported onthe plate 44 which forms a portion of the body of the leading dollysection 17. COmpression springs 43a resist movement of dolly section 18toward dolly section 17. Spaced plates 46 extend from plate 44 and astationary shaft or post 47 extends vertically between the plate. Freelyand independently rotatable on bearings 48 encircling the post 47 arethe sheaves 51a and 51b. These sheaves are aligned with sheaves 35a and35b respectively and form the multiple sheave assembly generallyindicated at 22 in FIG. 1.

Spaced beneath the plate 44 is a plate 52 and interposed between theplates is a shaft 53 which is manu ally rotatable by any suitable meanssuch as insertion of a tool into its head 53a. The shaft carries thetake-up winch 24, previously mentioned with reference to schematicFIG. 1. The winch is locked by insertion of a pin (not shown) inwhichever of the four apertures 54 in the winch register with theopenings 44a and 52a in the plates 44 and 52 when the proper adjustmentof the winch has been made. As previously mentioned, winch 24 might belocated on dolly section 18. A post 56 carries a rope thimble 57 aroundwhich the cable or rope may be secured. Wheel housings 19a carry thewheels 19 and are welded or otherwise rigidly secured to the tubes.

FIG. 4 illustrates the sectional dolly schematically and is utilized toillustrate in detail the reeving of the rope or cable upon the sheavesand the take-up winch. In FIG. 4 the dolly is shown in simplified formwith parts omitted and with the rotational axes of the sheaves and winchtransposed to generally horizontal, rather. than vertical, position sothe reeving and the blockand-tackle mode of operation of the dolly willbe evident. For clarity the same reference numerals are used forcorresponding parts in FIGS. 1, 2, 3, and 4, although the parts mayappear somewhat different in position or contour in the schematicshowing of FIGS. 1 and 4 compared to their appearance in FIGS. 2 and 3.

As previously mentioned and as shown in FIG. 4, the rope or cable isseparated and the sectional dolly is interposed in the separation withthe high tension side of the rope 12, the portion nearest the drivingpulley 10, being attached to dolly section 17 at 12a (the attachment ismade around thimble 57 on the actual dolly structure shown in FIG. 3).The low tension portion 12c of the rope, the portion nearest idler ortail sheave 15, extends past the trailing section 18 of the dolly andaround the grooved surface of sheave 51b. The rope extends from sheave51b to sheave 35b in a pass identified at 61 in FIG. 4. Leaving theunderside (as viewed in FIG. 4) of sheave 35b, the rope extends back todolly section 17 in a pass identified at 62. The rope then enters thegroove of sheave 51a at its underside (as viewed in FIG. 4) and extendsaround sheave 51a to exit at the top of sheave 51a and extend, in a passidentified at 63, to sheave 35a on trailing dolly section 18. The ropeextends around the grooved peripheral surface of sheave 35a and extendsfrom the lower side of the sheave to the leading dolly section 17 andpast sheave 51a is a pass identified at 64 to terminate by beingattached to the take-up winch 24, as indicated at 64a.

As will be evident from FIG. 4, the dolly sections and the reeving ofthe rope between them is analogous to a block and tackle connectionbetween the dolly sections. The member 26 picks up the load and thedolly section 18 is analogous to the movable block with multiple sheavesin a conventional block and tackle hoist; the dolly section 17 can, forpurposes of analysis be considered to be the fixed block of aconventional block and tackle hoist. As is well known in the mechanicsof block and tackle hoists a load picked up by member 26 can be movedfrom right to left, as viewed in FIG. 4, by exerting a pulling force onthe pull rope (for purposes of analysis considered to be rope portion ofl/n times the load, where n is the number of rope passes extending fromthe movable block (dolly section 18), and neglecting friction in thesheaves. Thus, continuing the analogy, if member 26 is to move a load W,a pull must be exerted on rope 120 of (n=4 in FIG. 4) of the load W.

Now deserting the analogy to the extent that dolly section 17 no longerbe considered stationary but mobile and fastened to the higher tensionside of the rope 12 (as, in fact, it is in FIGS. 4 and 1) when member 26engages a load W, the drive pulley assembly 10-11 (FIG. 1) will apply tothe system a force necessary to move the load if proper belt tension atT (FIG. 1) is maintained. As dolly section 17 starts to move leftwardly,as viewed in FIG. 1 and 4, the distance between dolly sections 17 and 18will increase slightly but eventually dolly section 18 and consequentlythe load, en-

gaged by member 26, will begin to move. As this occurs, since this is abalanced, stable system, a tensioning force will be transferred to thesmaller tension side of the rope (120) which is equal (approximately,neglecting friction) to A of the load (n=4). The block and tackle actionof the dolly automatically, upon the imposition of a load on the system,transfers to the smaller tension side of the rope, and thus to thepulley rope junction T, (FIG. 1), a predetermined fraction of the forcenecessary to move the load. As load movement proceeds the dolly section18 is separated from dolly section 17 by a distance which depends uponthe magnitude of the load.

In operation, the take-up winch 24 (FIG. 1) is initially adjusted totake up any excess cable and to draw the dolly section 18 toward dollysection 17 somewhat so that compression springs 43a (FIG. 2) arecompressed somewhat. This establishes a minimum initial tension on therope so that the system will produce sufficient frictional force betweenthe rope and the driving pulley assembly -11 to enable it to move thecable and dolly under no-load conditions, that is, when the member 26 isnot engaged with a load to be moved. It will be understood that, aspreviously mentioned, as the cable moves the dolly from right to left,that is, with the drive pulley assembly rotating counterclockwise asviewed in FIG. 1, the load pick-up member 26 will engage a suitableabutment on a loaded car (not shown) or container as it passes beneaththe car. When the drive is reversed, the member 26 will not engage aloaded container but will pivot and slip past the container or car. Thesystem illustrated in FIG. 1 thus is adapted to move loadsunidirectionally only.

When a load is engaged by member 26, as pointed out previously withreference to FIG. 4, the dolly section 18 will separate somewhat furtherfrom dolly section 17 and a fraction (approximately, neglectingfriction, A since n=4) of the force necessary to move the load exertedby the drive pulley assembly is transferred to the smaller tension side12b of the rope, thereby increasing the tension at T (FIG. 1) asrequired. The action of the dolly is such as to increase T as the loadincreases, thus automatically conditioning the system for accommodatingloads of various magnitudes.

A modified form of the dolly, utilizing three sections, is schematicallyillustrated in FIG. 5 and is adapted for bidirectional movement of aload, that is, movement of a load in either direction of movement of therope. The structure includes a central dolly section 71 which floatsbetween the dolly sections 72 and 73. The outer dolly section 72 and 73are urged apart by a compression spring schematically shown at 74 whichacts on a telescoping rod and tube assembly 76 carried jointly by thetwo outer dolly sections.

The two outer dolly sections 72 and 73 each carry a load engagingabutment 77 and 78, respectively. It will be understood that theabutments correspond in structure to the abutment 26 of FIG. 1 and thatabutment 77 is contoured and pivotally mounted so that it will pick uploads only when the dolly is moving from right to left as viewed in FIG.5, and abutment 78 will pick up loads only when the dolly is moving fromleft to right.

The rope or flexible cable 12 extends, as indicated at 12d, from thetail sheaves (not shown in FIG. 5), freely across dolly section 72 andaround a freely rotatable sheave 81 carried by central dolly section 71and back to freely rotatable sheave 82 carried by dolly section 72. Therope then passes around and between freely rotatable sheaves 83 and84,carried by dolly sections 71 and 72, respectively, in block and tacklefashion. The rope is accommodated on the take-up winch 86, whichcorresponds to take-up winch 22 of FIG. 1.

As indicated at 12e in FIG. 5 the rope 12 passes freely across the dollysection 73 and around a freely rotatable sheave 87 carried by dollysection 71. The rope then passes in block and tackle fashion betweensheaves 88 and 89 which are carried by dolly section 73, and the sheave91, mounted on the same rotational axis as sheave 87. Finally, the ropeis rigidly anchored to the central dolly section 71, as indicated at12f. It will be noted that the number of rope passes extending fromsheaves 82-84 and from sheaves 87-91 are both four so that, in eachcase, n equals four.

The operation of this three-section dolly is generally the same as thatdescribed with reference to FIGS. 1-4. The central dolly section 71changes its relative position with respect to the outer dolly sectionsdepending upon which of the two outer dolly sections has picked up theload. When the abutment 77 picks up the load, moving it from right toleft in FIG. 5, the central dolly section 71 will be pulled forwardagainst dolly section 73, as illustrated in FIG. 5, and a predeterminedfractional portion, approximating (neglecting sheave friction) 11/11, ofthe force necessary to move the load will be transferred to the smallertension side (12d) of the rope. Similarly, when the abutment 78 picks upthe load, the dolly section 71 will be drawn against the dolly section72. The side of the rope 12e, now the smaller tension member, will [havetransferred to it approximately a l/n fraction of the force necessary tomove the load.

In the foregoing description and in the claims in this application itwill be understood that approximately equals refers to neglect of sheavefriction in arriving at the values involved. While the load pick-upabutments are disclosed as directional-sensitive pivoted dogs, it willbe evident that these could take the form of manually actuated membersor remotely operated members, with, for example, electrical solenoidactuators. Rope as used herein is intended to include any flexiblemember such as a chain, cable, belt, or the like.

I claim:

1. A self-tensioning, endless rope drive system comprising rotatingdrive pulley means accommodating a rope on a portion of its peripheralsurface, the rope extending tangentially from the pulley means, thenaround a sheave spaced from the pulley means and returning tangentiallyto the pulley means so that as the drive pulley means rotates, oneportion of the rope moves onto said drive pulley means under hightension and another portion moves off said drive pulley means underlower tension, a load-engaging member carried by said rope and adaptedto be moved between said sheave and said pulley means, and forcetransferring means extending between said load-engaging member and saidrope for applying a predetermined portion of the driving force necessaryto move the load engaged by said member to said lower tension portion ofthe rope.

2. A self-tensioning endless. rope drive system comprising a rotatingdrive pulley assembly and an idler sheave spaced therefrom along a pathover which a load is to be moved, a rope extending from said pulleyassembly around said idler sheave and returning to said pulley assembly,whereby as said drive pulley assembly rotates said rope moves onto thepulley assembly under high tension and off said pulley assembly underlower tension, a sectionalized dolly having at least two generallyaligned segments spaced along and adapted to travel along said path overwhich a load is to be moved, said dolly being interposed within aseparation in the rope intermediate said path, the leading dolly sectionand the trailing dolly section carrying matching sheave assemblies, theportion of the rope adjacent said rope separation and nearest said idlersheave extending to the sheave assembly on the leading dolly section andthence in n passes between the leading dolly section sheave assembly andthe trailing dolly section sheave assembly, n being a whole integer, anda load engaging element carried by said trailing dolly section wherebyas said rope is moved by said drive pulley assembly and a load isengaged by said element an approximate l/n fraction of the forcenecessary to move the load is transferred to said lower tension side ofthe rope by the block and tackle action of the leading and trailingdolly section sheave assemblies.

3. A self-tensioning endless rope drive system as claimed in claim 2 inwhich said load engaging element is adapted to engage a load when saidelement is moved in one direction by said dolly but not when moved inthe opposite direction.

4. A self-tensioning endless rope drive system as claimed in claim 2 inwhich each of said dolly section sheave assemblies is formed by dualsheaves rotating on the same axis and n equals four.

5. A self-tensioning endless rope drive system as claimed in claim 2 inwhich resilient means urges said dolly sections apart to place a minimumtensioning force on the lower tension side of the rope.

6. A self-tensioning endless rope drive system as claimed in claim 5 inwhich said resilient means takes the form of telescoping members jointlycarried by said dolly sections with compression springs resistingtelescoping of the members.

7. A self-tensioning endless rope drive system as claimed in claim 2having adjusting means on said dolly for adjustably reducing theeffective length of said rope to compensate for permanent strengththereof after prolonged use and for extablishing a minimum tensioningforce on the lower tension side of the rope.

8. A self-tensioning endless rope drive system as claimed in claim 7 inwhich said adjusting means takes the form of a take-up winch carried onsaid dolly.

9. A self-tensioning endless rope drive system as claimed in claim 2 inwhich a load may be transported bidirectionally and in which said loadengaging element carried by said trailing dolly section is adapted toengage a load when said element is moved in one direction by said dollybut not when moved in the opposite direction and said dolly includes anadditional dolly section carrying a sheave assembly, and a furthersheave assembly carried by said leading dolly section matching saidadditional dolly section sheave assembly, the portion of the ropeadjacent said rope separation and nearest said drive pulley assemblyextending to said additional sheave assembly on said leading dollysection and thence in n passes between said further leading dollysection sheave assembly and said additional dolly section sheaveassembly, n being a whole integer, and an additional load engagingelement carried by said additional dolly section and adapted to engage aload only when said additional load engaging element is moved in saidopposite direction by said dolly, whereby a load can be moved in eitherdirection of movement of said rope with an approximate 1/n fraction ofthe force necessary to move the load in said one direction and anapproximate l/n' fraction of the force necessary to move the load insaid opposite direction being transferred to the lower tension side ofthe rope as determined by the direction of movement of the rope anddolly.

10. A self-tensioning endless rope drive system as claimed in claim 9 inwhich resilient means is provided for urging said additional dollysection and said trailing dolly section away from each other.

1. A self-tensioning, endless rope drive system comprising rotatingdrive pulley means accommodating a rope on a portion of its peripheralsurface, the rope extending tangentially from the pulley means, thenaround a sheave spaced from the pulley means and returning tangentiallyto the pulley means so that as the drive pulley means rotates, oneportion of the rope moves onto said drive pulley means under hightension and another portion moves off said drive pulley means underlower tension, a loadengaging member carried by said rope and adapted tobe moved between said sheave and said pulley means, and forcetransferring means extending between said load-engaging member and saidrope for applying a predetermined portion of the driving force necessaryto move the load engaged by said member to said lower tension portion ofthe rope.
 2. A self-tensioning endless rope drive system comprising arotating drive pulley assembly and an idler sheave spaced therefromalong a path over which a load is to be moved, a rope extending fromsaid pulley assembly around said idler sheave and returning to saidpulley assembly, whereby as said drive pulley assembly rotates said ropemoves onto the pulley assembly under high tension and off said pulleyassembly under lower tension, a sectionalized dolly having at least twogenerally aligned segments spaced along and adapted to travel along saidpath over which a load is to be moved, said dolly being interposedwithin a separation in the rope intermediate said path, the leadingdolly section and the trailing dolly section carrying matching sheaveassemblies, the portion of the rope adjacent said rope separation andnearest said idler sheave extending to the sheave assembly on theleading dolly section and thence in n passes between the leading dollysection sheave assembly and the trailing dolly section sheave assembly,n being a whole integer, and a load engaging element carried by saidtrailing dolly section whereby as said rope is moved by said drivepulley assembly and a load is engaged by said element an approximate 1/nfraction of the force necessary to move the load is transferred to saidlower tension side of the rope by the block and tackle action of theleading and trailing dolly section sheave assemblies.
 3. Aself-tensioning endless rope drive system as claimed in claim 2 in whichsaid load engaging element is adapted to engage a load when said elementis moved in one direction by said dolly but not when moved in theopposite direction.
 4. A self-tensioning endless rope drive system asclaimed in claim 2 in which each of said dolly section sheave assembliesis formed by dual sheaves rotating oN the same axis and n equals four.5. A self-tensioning endless rope drive system as claimed in claim 2 inwhich resilient means urges said dolly sections apart to place a minimumtensioning force on the lower tension side of the rope.
 6. Aself-tensioning endless rope drive system as claimed in claim 5 in whichsaid resilient means takes the form of telescoping members jointlycarried by said dolly sections with compression springs resistingtelescoping of the members.
 7. A self-tensioning endless rope drivesystem as claimed in claim 2 having adjusting means on said dolly foradjustably reducing the effective length of said rope to compensate forpermanent strength thereof after prolonged use and for extablishing aminimum tensioning force on the lower tension side of the rope.
 8. Aself-tensioning endless rope drive system as claimed in claim 7 in whichsaid adjusting means takes the form of a take-up winch carried on saiddolly.
 9. A self-tensioning endless rope drive system as claimed inclaim 2 in which a load may be transported bidirectionally and in whichsaid load engaging element carried by said trailing dolly section isadapted to engage a load when said element is moved in one direction bysaid dolly but not when moved in the opposite direction and said dollyincludes an additional dolly section carrying a sheave assembly, and afurther sheave assembly carried by said leading dolly section matchingsaid additional dolly section sheave assembly, the portion of the ropeadjacent said rope separation and nearest said drive pulley assemblyextending to said additional sheave assembly on said leading dollysection and thence in n'' passes between said further leading dollysection sheave assembly and said additional dolly section sheaveassembly, n'' being a whole integer, and an additional load engagingelement carried by said additional dolly section and adapted to engage aload only when said additional load engaging element is moved in saidopposite direction by said dolly, whereby a load can be moved in eitherdirection of movement of said rope with an approximate 1/n fraction ofthe force necessary to move the load in said one direction and anapproximate 1/n'' fraction of the force necessary to move the load insaid opposite direction being transferred to the lower tension side ofthe rope as determined by the direction of movement of the rope anddolly.
 10. A self-tensioning endless rope drive system as claimed inclaim 9 in which resilient means is provided for urging said additionaldolly section and said trailing dolly section away from each other.